Microtubule-associated proteins (MAPs) are main candidates to stabilize neuronal microtubules, playing an important role in establishing axon-dendrite polarity. However, how MAPs are selectively ...targeted to specific neuronal compartments remains poorly understood. Here, we show specific localization of microtubule-associated protein 6 (MAP6)/stable tubule-only polypeptide (STOP) throughout neuronal maturation and its role in axonal development. In unpolarized neurons, MAP6 is present at the Golgi complex and in secretory vesicles. As neurons mature, MAP6 is translocated to the proximal axon, where it binds and stabilizes microtubules. Further, we demonstrate that dynamic palmitoylation, mediated by the family of α/β Hydrolase domain-containing protein 17 (ABHD17A-C) depalmitoylating enzymes, controls shuttling of MAP6 between membranes and microtubules and is required for MAP6 retention in axons. We propose a model in which MAP6’s palmitoylation mediates microtubule stabilization, allows efficient organelle trafficking, and controls axon maturation in vitro and in situ.
•MAP6 localizes to the Golgi complex and secretory vesicles in unpolarized neurons•Palmitoylated MAP6 targets to the newly formed axon, where it stabilizes microtubules•ABHD17 depalmitoylating enzymes control MAP6 membrane-microtubule shuttling•MAP6 mediates organelle trafficking and axon maturation in vitro and in situ
Tortosa et al. show that MAP6 re-distributes from Golgi and secretory vesicles to axonal microtubules during neuronal polarization. Palmitoylation cycles control MAP6 membrane-microtubule shuttling. The authors also demonstrate the importance of MAP6 for microtubule stabilization, organelle trafficking, and axon maturation.
Although originally discovered as neuronal growth cone-collapsing factors, repulsive guidance molecules (RGMs) are now known as key players in many fundamental processes, such as cell migration, ...differentiation, iron homeostasis, and apoptosis, during the development and homeostasis of many tissues and organs, including the nervous, skeletal, and immune systems. Furthermore, three RGMs (RGMa, RGMb/DRAGON, and RGMc/hemojuvelin) have been linked to the pathogenesis of various disorders ranging from multiple sclerosis (MS) to cancer and juvenile hemochromatosis (JHH). While the molecular details of these (patho)biological effects and signaling modes have long remained unknown, recent studies unveil several exciting and novel aspects of RGM processing, ligand–receptor interactions, and downstream signaling. In this review, we highlight recent advances in the mechanisms-of-action and function of RGM proteins.
The semaphorins, originally discovered as evolutionarily conserved steering molecules for developing axons, also influence neuronal structure and function in the early postnatal and juvenile nervous ...systems through several refinement processes. Semaphorins control synaptogenesis, axon pruning, and the density and maturation of dendritic spines. In addition, semaphorins and their downstream signaling components regulate synaptic physiology and neuronal excitability in the mature hippocampus, and these proteins are also implicated in a number of developmental, psychiatric, and neurodegenerative disorders. Significant inroads have been made in defining the mechanisms by which semaphorins regulate dynamic changes in the neuronal cytoskeleton at the molecular and cellular levels during embryonic nervous system development. However, comparatively little is known about how semaphorins influence neuronal structure and synaptic plasticity during adult nervous system homeostasis or following injury and disease. A detailed understanding of how semaphorins function beyond initial phases of neural network assembly is revealing novel insights into key aspects of nervous system physiology and pathology.
Rab6 is a conserved small GTPase that localizes to the Golgi apparatus and cytoplasmic vesicles and controls transport and fusion of secretory carriers 1. Another Rab implicated in trafficking from ...the trans-Golgi to the plasma membrane is Rab8 2–5. Here we show that Rab8A stably associates with exocytotic vesicles in a Rab6-dependent manner. Rab8A function is not needed for budding or motility of exocytotic carriers but is required for their docking and fusion. These processes also depend on the Rab6-interacting cortical factor ELKS 1, suggesting that Rab8A and ELKS act in the same pathway. We show that Rab8A and ELKS can be linked by MICAL3, a member of the MICAL family of flavoprotein monooxygenases 6. Expression of a MICAL3 mutant with an inactive monooxygenase domain resulted in a strong accumulation of secretory vesicles that were docked at the cell cortex but failed to fuse with the plasma membrane, an effect that correlated with the strongly reduced mobility of MICAL3. We propose that the monooxygenase activity of MICAL3 is required to regulate its own turnover and the concomitant remodeling of vesicle-docking protein complexes in which it is engaged. Taken together, the results of our study illustrate cooperation of two Rab proteins in constitutive exocytosis and implicates a redox enzyme in this process.
► Rab8A associates with exocytotic vesicles in a Rab6-dependent manner ► Rab8A and MICAL3 participate in docking and fusion of exocytotic vesicles ► Rab8A and ELKS can be linked by the flavoprotein monooxygenase MICAL3 ► The turnover of MICAL3 is regulated by its monooxygenase activity
The brain endocannabinoid system plays a crucial role in emotional processes. We have previously identified an important role for endocannabinoids in social play behavior, a highly rewarding form of ...social interaction in adolescent rats. Here, we tested the hypothesis that endocannabinoid modulation of social play behavior occurs in brain regions implicated in emotion and motivation. Social play increased levels of the endocannabinoid anandamide in the amygdala and nucleus accumbens (NAc), but not in prefrontal cortex or hippocampus of 4- to 5-week-old male Wistar rats. Furthermore, social play increased phosphorylation of CB1 cannabinoid receptors in the amygdala. Systemic administration of the anandamide hydrolysis inhibitor URB597 increased social play behavior, and augmented the associated elevation in anandamide levels in the amygdala, but not the NAc. Infusion of URB597 into the basolateral amygdala (BLA) increased social play behavior, and blockade of BLA CB1 cannabinoid receptors with the antagonist/inverse agonist SR141716A prevented the play-enhancing effects of systemic administration of URB597. Infusion of URB597 into the NAc also increased social play, but blockade of NAc CB1 cannabinoid receptors did not antagonize the play-enhancing effects of systemic URB597 treatment. Last, SR141716A did not affect social play after infusion into the core and shell subregions of the NAc, while it reduced social play when infused into the BLA. These data show that increased anandamide signaling in the amygdala and NAc augments social play, and identify the BLA as a prominent site of action for endocannabinoids to modulate the rewarding properties of social interactions in adolescent rats.
Following the decline of neurogenesis at birth, progenitors of the subventricular zone (SVZ) remain mostly in a quiescent state in the adult human brain. The mechanisms that regulate this quiescent ...state are still unclear. Here, we isolate CD271
progenitors from the aged human SVZ for single-cell RNA sequencing analysis. Our transcriptome data reveal the identity of progenitors of the aged human SVZ as late oligodendrocyte progenitor cells. We identify the Wnt pathway antagonist SFRP1 as a possible signal that promotes quiescence of progenitors from the aged human SVZ. Administration of WAY-316606, a small molecule that inhibits SFRP1 function, stimulates activation of neural stem cells both in vitro and in vivo under homeostatic conditions. Our data unravel a possible mechanism through which progenitors of the adult human SVZ are maintained in a quiescent state and a potential target for stimulating progenitors to re-activate.
Many guidance receptors are proteolytically cleaved by membrane-associated metalloproteases of the ADAM family, leading to the shedding of their ectodomains. Ectodomain shedding is crucial for ...receptor signaling and function, but how this process is controlled in neurons remains poorly understood. Here, we show that the transmembrane protein Lrig2 negatively regulates ADAM-mediated guidance receptor proteolysis in neurons. Lrig2 binds Neogenin, a receptor for repulsive guidance molecules (RGMs), and prevents premature Neogenin shedding by ADAM17 (TACE). RGMa reduces Lrig2-Neogenin interactions, providing ADAM17 access to Neogenin and allowing this protease to induce ectodomain shedding. Regulation of ADAM17-mediated Neogenin cleavage by Lrig2 is required for neurite growth inhibition by RGMa in vitro and for cortical neuron migration in vivo. Furthermore, knockdown of Lrig2 significantly improves CNS axon regeneration. Together, our data identify a unique ligand-gated mechanism to control receptor shedding by ADAMs and reveal functions for Lrigs in neuron migration and regenerative failure.
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•Lrig2 negatively regulates ectodomain shedding of Neogenin by ADAM17•RGMa inhibits Lrig2-Neogenin binding, allowing ADAM17-mediated cleavage of Neogenin•Lrig2 controls neuron migration, and Lrig2 knockdown improves axon regeneration•Lrig2 inhibits ectodomain shedding of multiple, distinct ADAM17 substrates
How proteolytic cleavage of cell-surface proteins is controlled in neurons is incompletely understood. Van Erp and van den Heuvel et al. show that Lrig2 negatively regulates ADAM17-mediated ectodomain shedding of the guidance receptor Neogenin. This process is required for proper neuron migration during embryonic development and during axon regeneration.
Myelin-associated glycoprotein (MAG) is a myelin-expressed cell-adhesion and bi-directional signalling molecule. MAG maintains the myelin-axon spacing by interacting with specific neuronal ...glycolipids (gangliosides), inhibits axon regeneration and controls myelin formation. The mechanisms underlying MAG adhesion and signalling are unresolved. We present crystal structures of the MAG full ectodomain, which reveal an extended conformation of five Ig domains and a homodimeric arrangement involving membrane-proximal domains Ig4 and Ig5. MAG-oligosaccharide complex structures and biophysical assays show how MAG engages axonal gangliosides at domain Ig1. Two post-translational modifications were identified-N-linked glycosylation at the dimerization interface and tryptophan C-mannosylation proximal to the ganglioside binding site-that appear to have regulatory functions. Structure-guided mutations and neurite outgrowth assays demonstrate MAG dimerization and carbohydrate recognition are essential for its regeneration-inhibiting properties. The combination of trans ganglioside binding and cis homodimerization explains how MAG maintains the myelin-axon spacing and provides a mechanism for MAG-mediated bi-directional signalling.
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